1. Field of the Invention
The present invention relates generally to the field of ohmic contacts for semiconductor electronic devices and, more particularly, to a low-penetration ohmic contact providing a gettering function to remove impurities from semiconductor devices and to a method of forming it.
2. Description of the Related Art
In recent years, communications systems have been developed in many parts of the world that take advantage of optical fiber technology to increase the speed of information transfer. Demand for ever-higher information transfer rates has resulted in pressure to advance the state of the art of optoelectronic communications technology, including the design and manufacturing of semiconductor lasers.
Semiconductor lasers are the power sources for optoelectronic communications. They transform electronic signals into light with specific properties of intensity and spectral purity designed to allow for transmission of information over optical fiber networks. Although prior art semiconductors most commonly employed the elements Si and Ge, listed in Group IV of the Periodic Table, other materials, such as intermetallic compounds, have also been found to exhibit properties useful in the formation of semiconductor lasers.
Modern semiconductor lasers are precision devices in which many layers of semiconductors are used to control the flow of current and light. The device may have as many as twenty or more semiconductor substructures and require up to 4 or 5 crystal growth steps. The active region of the laser, the area in which the conversion of electrons and holes to photons occurs, may be made up of a number of substructures called quantum wells, some of which may be only 10 atoms thick.
In order to provide high efficiency of conversion of electrical to optical power, the electrical fields inside the active region should be controlled. This implies a high degree of control of the dopant impurities in the neighborhood of the active region. Impurities deliberately introduced into the semiconductor to modify the carrier density and control conductivity type should be controlled with respect to position and density to a high degree of spatial precisionxe2x80x94a few nanometers (nm).
The contact metallization, or the part of the laser that interfaces with the outside world to provide chemical barriers and electrical contacts, has generally received less attention in design efforts than the active region itself. Designs for the contact structure have been nearly frozen for a long time. However, in recent years it has become apparent that the control of dopant diffusion into the active region requires that the other layers be optimized with respect to thickness and doping density. This has in turn placed additional requirements on the contact metallization, or ohmic contact.
Ohmic contacts to semiconductor devices are the point at which the outside world in the form of electricity flowing in wires influences the semiconductor. In addition to the obvious function of conducting electricity, ohmic contacts provide chemical isolation of the active region from impurities that can affect device performance. These impurities are often generated by further processing of the device during manufacturing or during normal usage. Ohmic contacts also provide an important path for heat to escape from the active region, and they may also be required to provide mechanical isolation of the device during bonding processes. To perform these functions, the contact structure applied to the semiconductor may be comprised of many layers, all of which should be compatible with each other and with the semiconductors and other elements of the device.
In modern ohmic contacts, the ohmic function is provided by one or a few layers of metal and the other functions (chemical barrier, mechanical protection, etc.) are provided by other layers specifically aimed at those purposes. Ohmic metallization recipes of the xe2x80x9calloyedxe2x80x9d type incorporate a solvent metal such as Au which reacts strongly with the semiconductor and a dopant element such as Sn or Ge for N-type III-V materials and Be or Zn for P-type materials. The purpose of the solvent is to break up the semiconductor and form a strong physical bond. The purpose of the dopant is to increase the concentration of acceptors or donors in the immediate neighborhood of the interface and thereby reduce the contact resistance. Such an xe2x80x9calloyedxe2x80x9d ohmic contact recipe is primarily designed to optimize contact resistance.
The classic ohmic contact used in many current optoelectronic devices is produced using an alloyed ohmic contact recipe and consists of Au and Be. This contact is used for P-type materials and consists of an alloy of Au and Be which is evaporated as a single film from a source of mixed metal. The Au is a solvent and the Be a P-type dopant. The vapor pressures of Au and Be are similar enough to allow material of essentially constant composition to be applied by physical evaporation, typically in an vacuum chamber at pressures below 5xc3x9710xe2x88x927 Torr using an electron-beam gun as a power source. The AuBe alloy is applied to the semiconductor through a patterned photoresist mask and the excess metal lifted off when the photoresist is dissolved in acetone. The AuBe layer is then annealed (heated) at a temperature of 350-420 degrees Celsius to form the metallurgical bond and other metal layers are applied in subsequent fabrication steps to form the protective barrier and mechanical bond layers.
At the end of the complete metallization process, the metals in the stack may comprise up to 5 microns of metal in 7 to 10 distinct layers. For example, a typical metallization might have the sequence: AuBe Ti Pt Au Au Pt Au (i.e., AuBe is first deposited, Ti is deposited on the AuBe, Pt is deposited on the Ti, etc.), in which the AuBe layer functions as the ohmic contact interface to the semiconductor, the sequence Ti Pt Au functions as the barrier metallization for chemical protection, and the sequence Au Pt Au functions as the bonding layer for mechanical protection. It is desirable to use Au as a final layer of a metal sequence which will be exposed to air or processing materials. This is because Au is the least reactive and most easily cleaned metal.
Between the semiconductor and the metal layers, a layer of low-bandgap, lattice-matched semiconductor is often applied as the final xe2x80x9ccap layerxe2x80x9d interface to reduce the Schottky barrier height and further facilitate the contact. In the case of the InGaAsP alloy system, the lattice-matched material is InGaAs. An additional benefit of using InGaAs as the cap layer is that it has a higher solubility for the preferred P-dopant Zn than does InP.
However, the use of an InGaAs cap layer presents a hazard of introducing additional impurities into the active region of the semiconductor. Uncontrolled diffusion of Zn, an impurity, in the neighborhood of the active region occurs mainly during the growth of the InGaAs cap layer. Therefore, it is desirable to minimize the amount of time spent forming the cap layer, thus reducing the desired cap layer thickness. In the past, most laser devices used a 500 nm thick cap layer. Recently, this was reduced to 200 nm and more recently to a minimum of 50 nm.
Because of the concern to reduce the thickness of the InGaAs cap layer, development efforts were aimed at producing a xe2x80x9clow penetration contactxe2x80x9d which was designed to react with a controlled minimum amount of semiconductor. The depth of reaction with the semiconductor is controlled by the thickness of the layers of metallization which interacted directly with the semiconductor. The idea is that the complete reaction of the layers of metal which combine with the semiconductor consumes only a small amount of semiconductor cap layer material. In order to make sure that the total penetration of the metal is kept to a desired limit, it is necessary to use very thin layers at the semiconductor/metal interface. For purposes of manufacturability, it is desirable that the very thin layers be made of single elements, not alloys, mixtures, or compounds.
The classic AuBe ohmic contact was found to be unsuitable for use with thin (approximately 50 nm) InGaAs cap layers. The depth of reaction of AuBe with the cap layer is highly variable, sometimes resulting in excess penetration of AuBe into the underlying InP substrate. Thus, a thick cap layer of about 200 nm or more is usually required for satisfactory reliability of the AuBe contact.
In addition to the Zn impurity hazard, Copper (Cu) is an even more dangerous impurity for InGaAsP semiconductor devices. Even at very low levels, Cu ions act as efficient destroyers of electrons and holes via a process called non-radiative recombination. Cu also diffuses rapidly through the semiconductor. Cu is most often introduced as an impurity in the contact metallization, either in the original AuBe source material or due to processing steps. Thus it is desirable to use metallization source materials unlikely to contain Cu impurities. Cu impurities are often present in the AuBe source metal, degrading reliability of the resulting semiconductor device and requiring extensive composition control of the alloy during contact formation. These problems are distinctly alloy-related; they are not significant in contact recipes using only pure metal layers.
An ohmic contact recipe containing pure metal layers was proposed for use with thin InGaAs cap layers in U.S. patent application Ser. No. 09/017,103, which is hereby incorporated by reference herein. In this application, a contact metallization with sequence Pt Ti Pt Au Ti Pt Au Au Pt Au was used and tested with a 50 nm thick InGaAs cap layer on a III-V semiconductor device. This metallization sequence effectively replaces the AuBe alloy with a sequence of pure metal layers: Pt Ti Pt Au. The resulting contact showed excellent contact resistivity, very low depth of reaction into the cap layer, and was thought to contain few impurities.
However, detailed analysis of this contact metallization suggested that the resulting device may be further improved. Secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy showed indications that Zn impurities in the InGaAs cap layer, which were deliberately introduced as a dopant during epitaxial growth of the cap layer, did not appear to migrate upward from the p-type semiconductor material, including the cap layer, as may have happened in the AuBe layer of the classic AuBe contact during alloying. Improvements in device reliability could be obtained if Zn migration into the pure metal layers could be encouraged with this metallization.
Therefore, there is a desire and need for an ohmic contact recipe compatible with thin (50 nm) InGaAs cap layers, containing few impurities, exhibiting minimal resistivity, made of single elements capable of being manufactured in extremely thin layers, and resulting in devices showing improved reliability characteristics.
The present invention provides a novel ohmic contact metallization structure and method utilizing a thin pure metal layer designed to react with the semiconductor cap layer to getter Zn and other interstitial impurities. The resulting contact structure exhibits favorable reliability and resistivity characteristics and enables a higher level of integration for semiconductor devices by permitting the use of very thin layers in a low-penetration contact structure.
The present invention also provides an alternative to the prior art design scheme for ohmic contact structures. Prior art contact structure design was primarily concerned with minimizing contact resistance. The present invention provides an improved ohmic contact design scheme in which device reliability is the primary design goal, rather than minimal contact resistance. An ohmic contact metallization recipe showing improved reliability, excellent resistivity, and increased levels of integration can be obtained using the present invention.
The above and other features and advantages of the invention are achieved by forming a multilayer gettering contact metallization including a thin layer of a metal as the initial layer formed on the semiconductor cap layer for trapping impurities, dopants and self-interstitials. During formation of the contact structure, impurities, self-interstitials and other defects present in the cap layer and in other nearby layers diffuse into metal layers forming the contact structure, preventing further migration into active areas of the semiconductor device. The contact metallization is formed of pure metal layers compatible with each other and with the underlying semiconductor cap layer such that depth of reaction is minimized (low-penetration) and controllable by the thickness of the metal layers applied. Thin semiconductor cap layers, such as InGaAs cap layers less than 200 nm thick, may be used in the present invention with extremely thin pure metal layers of thickness 10 nm or less, thus enabling an increased level of integration for semiconductor optoelectronic devices. In addition, because pure metal layers are used in the ohmic contact, fewer impurities are introduced in the formation of the contact than with prior art alloy contacts.